This invention relates generally to a crystal growing method and more particularly to a crystal growing method for growing single crystals of material such as silicon in which the oxygen concentration gradient in the crystals is controlled.
The production of single crystals from materials such as silicon plays an important role in semiconductor technologies. A suitable method for growing the crystals is known as the Czochralski technique in which a seed crystal, which has the desired crystal orientation, is introduced into a melt of the semiconductor material. The melt can also contain certain dopants which are introduced for the purpose of modifying the electrical characteristics of the semiconductor material as is known in the art. The melt is contained in a silica crucible or vessel which is heated so that the semiconductor melt is at, or slightly above, its melting point. The seed crystal is slowly withdrawn from the melt, in an inert atmosphere such as argon, and the semiconductor material solidifies on the seed to produce the growth of a single crystal. A cylindrical crystal is produced by rotating the crystal as it is drawn. The crucible is conventionally rotated in the opposite direction for the purpose of mixing the melt under the growing crystal. The withdrawing rate and power to the heating means is greater at first in order to cause a neck down of the crystal. This reduces dislocations caused by the thermal shock which occurs when the seed crystal initially contacts the melt. The withdrawing rate is then decreased and the power is reduced in order to cause the diameter of the crystal to increase in a cone shaped manner until the desired crystal diameter is reached. The withdrawal rate and heating are then maintained constant until close to the end of the process where again the rate and heating are increased so that the diameter decreases to form a cone and neck portion at the end.
At the melt temperature of silicon (about 1,400.degree. C.), the surface of the silica crucible which is in contact with the melt dissolves and forms silicon monoxide, SiO, which enters the melt and evaporates from the surface of the melt. The SiO is a source of oxygen which enters the melt and, consequently the drawn crystal. Heretofore, the presence of oxygen in the crystal has been generally regarded as an undesirable impurity. It is also found that the oxygen concentration in the crystal is not constant but varies from the seed end, where it is at the highest level, to the tail end of the crystal where it is at its lowest level. Initially, the oxygen content of the melt is in the order of 3.times.10.sup.18 atoms per cubic centimeters which is about the saturation point. The oxygen in the grown crystal ranges from about 1.5.times.10.sup.18 atoms/cc in the seed down to around 6.times.10.sup.17 atoms/cc at the tail. From this, it is apparent that the oxygen content of the melt is depleted during the crystal growing process, probably due to a lower dissolution rate of the crucible as the process continues.
It has recently been found that the presence of oxygen can have beneficial effects on the properties of semiconductor devices manufactured from the grown crystals. For example, a reduction in leakage currents is noted at higher oxygen levels. Accordingly, it has been found that the observed beneficial effect on leakage occurs primarily with devices made from semiconductor wafers cut from the seed end of the crystal which has the higher oxygen contents. Therefore, it is desirable to be able to reduce the oxygen concentration gradient through the length of the crystal so that the same beneficial effects on the yield of devices will be obtained whether the semiconductor wafers used in making the devices are cut from the seed or the tail end of the crystal. A process to both increase the oxygen level in the crystals and to control the seed to tail oxygen concentration gradient is the subject of copending application Ser. No. 581,307, filed 5/27/75, entitled "Process for Controlling the Oxygen Content of Silicon Crystals", which involves the treatment of the crucible surface.
We have found another method for controlling the oxygen content. The method can either be used by itself or in combination with the method of Ser. No. 581,307.